Spring is usually a coordinated dance of singing birds, bursting leaves, buzzing insects, and blooming flowers, but climate change is throwing off the rhythm. With warming temperatures, the average spring in the eastern U.S. is starting earlier. This year, flowers bloomed 10 days early in Jackson, Mississippi, and leaves emerged up to 20 days early in West Virginia, according to the USA National Phenology Network.
Not every species is getting the memo.
For some plants and animals, temperature is the most important sign that spring is beginning, and when spring-like temperatures start early, so do they. Other species depend on different cues or a combination of cues – like the length of light in a day or the number of chilly days – to know when to start their spring. Those signals aren’t all affected by climate change in the same way, and some, like day length, aren’t affected at all. This leads to some species starting their springs earlier, while others stick closer to their historical schedules. That puts interspecies relationships – like those between plants and pollinators or predators and prey – at risk of falling out of sync.
Theresa Crimmins, the assistant director of USA National Phenology Network, said that in general, species that are adapting more quickly are increasing in abundance. “The ones that are not as responsive,” Crimmins said, are “tending to decrease in abundance or even be disappearing from the landscape.”
So which species are adapting faster and which are struggling? Here are five examples.
1. Winner: Invasive shrubs taking over forests
Many invasive shrubs, like certain honeysuckles, are taking advantage of one such timing mismatch. Researchers have discovered that in forests in the eastern U.S., invasive shrubs put out their leaves earlier in spring than their native counterparts like dogwoods, allowing them to capture sunlight with less competition. Erynn Maynard, a doctoral student at Penn State University, created a crowd-sourced project to track first leaf-out of invasive shrubs and native shrubs. For all of the invasive species but one, leaves emerged earlier than on the natives.
Maynard found that the gap between invasives and natives is particularly pronounced in the warmer Southeast but hypothesizes that it will increase in the Northeast as temperatures warm. Invasive shrubs, Maynard said, have also shown more resilience during false springs, when early warm temperatures break plants out of dormancy only for cold temperatures to return. In these cases, while natives were damaged, their invasive counterparts didn’t even lose their leaves.
An early start gives invasive shrubs energy to grow their populations and drown out others in their shade. This changes the types and numbers of plant species in the forest, which Maynard said has far-reaching impacts on plants and animals that researchers are just beginning to understand.
2. Loser: Woodland wildflowers losing light
Caitlin McDonough MacKenzie, a researcher at the Climate Change Institute at the University of Maine, discovered that woodland wildflowers in Massachusetts are responding to earlier springs more slowly than the tree canopy above them. By comparing first leaf-out data collected by Henry David Thoreau with modern records, McDonough MacKenzie found that in Concord, Massachusetts, trees’ leaves appear, on average, two weeks earlier than they did in the 1850s, while wildflowers have advanced their leaf-out less than one week in the same time period.
Having time to soak up sunlight before tree leaves emerge is essential for understory wildflowers, many of which gather anywhere from 60 to 80 percent of their annual energy in those early days of the year, said Mason Heberling, an assistant curator of botany at the Carnegie Museum of Natural History, with whom McDonough MacKenzie collaborated on this research.
McDonough MacKenzie said just under a quarter of wildflower species in Thoreau’s forests have either declined in abundance or disappeared since the philosopher’s time, but it’s hard to pinpoint the exact reason why. Increasing development, larger deer populations and decreased logging play a role in addition to climate change. But one thing is certain for these wildflowers: “It’s not going to get better,” she said.
3. Loser: Long-distance migratory birds flying too late
Birds that spend their winters in tropical South and Central America, far from their summer breeding grounds in the U.S. and Canada, often use day length as a cue to migrate. As early spring becomes more common, but days lengthen on the same schedule, some species are arriving at their breeding grounds late. A 2017 study by researchers from Canada and the U.S. found that while most eastern birds are adapting their migration timing, they are still lagging behind plants. The gap between plants leafing and birds arriving grew from 2001 to 2012.
Stephen Mayor, the study’s lead author and now an ecologist with the Ontario Ministry of Natural Resources and Forestry, said that he and colleagues used plants leafing as a sign of insect emergence. Birds historically have synced their breeding schedules to coincide with access to ideal food sources like caterpillars. Arriving too late may mean they miss out on their favorite meal. Finding an open and safe place to nest may also be made more difficult by arriving late. These factors, according to Mayor’s study, could harm the health of individual birds, reduce populations and, in the extreme case, cause local extinction.
Mayor said that birds experiencing significant mismatches include the scarlet tanager, indigo bunting, great crested flycatcher, eastern wood-pewee, rose-breasted grosbeak, blue-winged warbler, northern parula, yellow-billed cuckoo, and Townsend’s warbler.
Crimmins at the USA National Phenology Network added that the delayed arrival of migratory birds that eat insects could lead to booming insect populations that can defoliate trees and spread disease.
4. Winner: Short-distance migratory birds keeping pace
When the distance between a bird’s overwintering home and their breeding grounds is shorter, temperature cues can drive their migration. According to Stanley Temple, senior fellow at the Aldo Leopold Foundation, birds that have a shorter distance to migrate seem to be able to keep up with warming springs better.
For example, in southern Wisconsin, historical records from ecologist Aldo Leopold compared to current data have shown the short-migrant eastern phoebe has kept pace with an advancing spring. Most eastern phoebes overwinter in the southeastern U.S. Males return to Wisconsin early in the spring to claim nesting ground. The birds hang out around the early-emerging skunk cabbage plant, which has an odor that attracts insects and warmth that keeps the birds alive when it’s cold.
Between 1935 and 1945, Leopold documented that skunk cabbage’s average first bloom came on April 1, and the phoebes began to arrive April 7. Now, Temple said, skunk cabbage blooms March 18, on average, and the first phoebes arrive around March 24 – keeping the relationship intact despite the shifting season.
5. Loser: People planning to catch peak bloom
Normal weather variations make it hard enough for people trying to sync their schedule to a plant, like the cherry trees that bloom each year in Washington, D.C. A shift in the arrival of spring just makes the problem worse. Yet according to Lillian Iverson, the senior director of events and marketing for the National Cherry Blossom Festival, an estimated 1.5 million people turn up each year to participate in events celebrating the gift of 3,000 cherry trees from the city of Tokyo to Washington, D.C.
Many visitors plan their trips hoping to catch the trees at the peak of their blooming period. In 2018, February in D.C. was so warm that National Parks Service officials, who manage the trees, warned festival organizers to expect an early bloom. Organizers rescheduled the opening of their welcome area for two weeks earlier.
But then: “March came in like a lion with really cold weather,” Iverson said. “The trees didn’t bloom for another 14 days after their initial predicted date, so it was like, ‘All right, just kidding.’”
Iverson and the rest of the organizers are working to adapt to unpredictable springs. In 2012 – the 100-year anniversary of the trees’ arrival in D.C. – the festival expanded from 16 days to four weekends beginning on the first day of spring. In the years since, the team decided to stick with the extended timeline to get a better chance of matching up with peak blooms each year. They also make sure that each weekend is packed with events celebrating spring, so that there’s always something for visitors to do, whether they get their timing perfect or not.
Still more to learn
As these examples show, as plants and animals respond to climate change at different rates, there’s an increased risk that the delicate relationships between species will be broken. The implications of those breakups are likely to be wide-reaching – and have become a hot topic for researchers.
“We can kind of make these blanket statements about what seems to be happening,” Crimmins said, “but when you start to talk about within a community, how will things play out, and which species will stand to gain by conditions changing in a certain way, and which ones will lose out, it’s still complicated. It’s really hard to sort that all out.”